Caenibacterium thermophilum is a later synonym of Schlegelella thermodepolymerans

Many bacteria from diverse taxonomic groups accumulate polyhydroxyalkanoates (PHAs) as storage compounds for carbon and energy under unbalanced growth conditions (Schlegel et al., 1961; Anderson & Dawes, 1990). The most abundant type of PHA is poly(3-hydroxybutyrate), which was described in 1926 (Lemoigne, 1926); now, more than 140 different PHA constituents are known (Steinbüchel & Valentin, 1995). Because of the beneficial material properties of PHAs, they have attracted a lot of interest from academia and industry in the context of a variety of technical applications (Steinbüchel, 1991; Hocking & Marchessault, 1994). In general, PHAs are water-insoluble, thermoplastic and/or elastomeric, enantiomerically pure, non-toxic, biocompatible and, in particular, biodegradable (Doi & Steinbüchel, 2001).

Naturally, PHAs are released to the environment after cell lysis, and a large variety of PHA-utilizing micro-organisms occur in many ecosystems such as soil or compost (Delafield et al., 1965; Jendrossek et al., 1996). PHA-degrading micro-organisms excrete specific hydrolysing enzymes; many of these extracellular PHA depolymerases have been genetically and biochemically characterized (Jendrossek & Handrick, 2002). However, most studies have involved mesophilic micro-organisms and enzymes, so little is known about the biodegradation of PHAs at elevated temperatures.

Takeda et al. (1998) were the first to report on a thermotolerant and thermostable PHA depolymerase from a bacterium growing optimally at 45–50 °C. A thermophilic strain, HS^T, was isolated from hot-spring water and initially affiliated to the genus Leptothrix (Takeda et al., 1998, 2000). Later, it was taxonomically characterized in detail and described as Caldimonas manganoxidans gen. nov., sp. nov. (Takeda et al., 2002).

In a recent study, we isolated a novel bacterium from activated sludge under thermophilic growth conditions that was characterized as Schlegelella thermodepolymerans gen. nov., sp. nov. (Elbanna et al., 2003). This strain was capable of degrading poly(3-hydroxybutyrate) as well as the 3-mercaptopropionate-containing copolymer poly(3-hydroxybutyrate-co-3-mercaptopropionate). The latter was synthesized by PHA-accumulating bacteria, representing the first example of a novel class of biopolymer, which was referred to as polythioester (Lütke-Eversloh et al., 2001; Lütke-Eversloh & Steinbüchel, 2004). Furthermore, the extracellular thermostable PHA depolymerase of S. thermodepolymerans was purified and biochemically characterized: it was found to have substrate specificity for the oxoester linkages of PHAs, because thioesters could not be hydrolysed by this type of enzyme (Elbanna et al., 2004).

Interestingly, the polyester-degrading bacterium Caenibacterium thermophilum gen. nov., sp. nov., which was isolated from a thermophilic municipal sludge digester, was investigated at the same time, in parallel, in a different laboratory (Manaia et al., 2003). Therefore, Caenibacterium thermophilum and S. thermodepolymerans could be compared phylogenetically only after the published data became available (presented here). Both independently isolated bacteria showed significant similarities and, because of the date of valid publication of the names, we propose that Caenibacterium thermophilum is a later synonym of S. thermodepolymerans.

The 16S rRNA gene sequences were analysed by using the program blast (National Center for Biotechnology Information, ). The consensus sequences of strains belonging to the same phylogenetic group and of other representatives of the β-Proteobacteria (retrieved from the EMBL database) were aligned using the computer program clustal_x (Thompson et al., 1997). The resulting phylogenetic tree was calculated using the neighbour-joining method (Saitou & Nei, 1987) and displayed with TreeView (Page, 1996), using Wautersia metallidurans as an outgroup. Analysis of the 16S rRNA gene sequence of Caenibacterium thermophilum N2-680^T revealed 99·8 and 99·6 % similarity to the 16S rRNA gene sequences of strains K14^T and DhA-71, respectively, of S. thermodepolymerans, indicating that these bacteria belong to the same species. Regarding the nearest phylogenetic neighbours of the β-subclass of the Proteobacteria, S. thermodepolymerans is related to Leptothrix, Rubrivivax, Ideonella, Roseateles and Aquabacterium (Fig. 1⇓). Interestingly, S. thermodepolymerans exhibited 94 % similarity to Caldimonas manganoxidans, a recently described thermophilic bacterium that also degrades poly(3-hydroxybutyrate) (Takeda et al., 2002).

DNA–DNA hybridizations were carried out with photobiotin-labelled probes in microplate wells as described by Ezaki et al. (1989), using an HTS7000 Bio Assay Reader (Perkin Elmer) measuring fluorescence. The hybridization temperature was 50 °C. Reciprocal experiments were performed for every pair of strains and the values were interpreted according to the recommendations of Wayne et al. (1987). DNA–DNA hybridization confirmed that strain N2-680^T is closely related to K14^T and DhA-71, there being mean DNA reassociation levels of 97–98 % among the three strains.